Flexible waveguide



E. H. JoHNsoN ETAL 3,090,019

May 14, 1963 FLEXIBLE WAVEGUIDE Filed Feb. 24, 1959 United States Patent O ors to Andrew Corporation, Chicago, Ill., a corporation of illinois Filed Feb. 24, 1959, Ser. No. '795,125 7 Claims. (Cl. S33-9S) This invention relates to waveguides for use at microwave frequencies, and more specifically to a novel construction for flexible circular waveguides.

The advantages of use of waveguide over coaxial line of comparable size at frequencies greater than 3,000 rnegacycles or so are well-known, lying primarily in reduction of attenuation and increase of power-handling capacity for -any given size. ilu addition, of course, waveguide has the advantage of simplicity of construction because of the elimination of the dielectric and central conductor used in coaxial transmission lines. This advantage, however, in many cases is merely a theoretical one because `of practical considerations in the construction of over-all transmission systems. Coaxial cable offers the great advantage of flexibility, thus largely eliminating the problems of plumbing associated with ythe installation of conventional types of hollow waveguide. Where rigid waveguide is employed, it becomes necessary, in general, to fabricate special waveguide sections for each particular installation problem, because of the limitations on the radius :of curvature which can be tolerated Without introducing excessive attenuation. The construction of waveguide system-s between two points (a feed point and a load point) ychosen for reasons other than adaptability to standar lengths of straight and curved waveguide becomes difficult, if not impossible, without the fabrication of special lengths and curvatures of waveguide to lit the particular installation.

The problem 'thus introduced is further complicated by the necessity of great care in preventing discontinuities in electromagnetic characteristics on the interior of the guide, since any electromagnetic discontinuity along the length of the guide, whether of mechanical obstruction or conductivity change will act as a rellection or absorption point for the energy in the guide, thu-s greatly increasing the attenuation. For this reason, field fabrication of special lengths and curvatures `of rigid guide is extremely difticu-lt and expensive because of the necessity of extreme smoothness or uniformity along all portions of the internal structure of the guide, any solder, dent, etc., appearing on the inside of the guide .introducing substantail diminution of the theoretical performance of the guide as compared with a coaxial cable. Because of such consider-aions, coaxial cable must frequently be employed in place of waveguide for all or part of a high frequency transmission system ldespite its theoretical inferiority.

To deal with this limitation on the use of waveguides in high frequency transmission systems, there have heretofore been devised various forms of flexible waveguides. Su-ch waveguides, as heretofore devised, however, have been subject to serious objections. Rectangular waveguide, which is well-known to have very desirable properties both as regards attenuation :and preservation of the direction of polarization of the transmitted wave, is extremely expensive to fabricate in the form of ilexible waveguide. On the :other hand, `circular waveguide, although more simply fabricated, has heretofore introduced :other serious problems, particularly that of maintenance of polarization.

Because :of the circular symmetry of such waveguide, the propagated wave is readily caused :to `,follow a spiral path down the length :of the guide. Even -in straight cir- ICC cular waveguide, maintenance of the direction of polarization throughout the length of the guide constitutes a substantial problem, since even minor discontinuities on the interior of the guide, which are insutiic-ent in magnitude to introduce substantial attenuation problems, nevertheless upset the polarization pattern. When the guide is curved, particularly where the curve is not carefully planned, as in the case of a flexible guide, the problem of polarization pattern rotation becomes more severe. Because of this problem of alternation of the direction of polarization, it is common to employ, in circular waveguide, modes of transmission which are themselves of circular symmetry, such as the TEM and TMm, in which such rotation, a1- though existing in theory, produces no practical effects. The use of these modes, however, has, other factors being equal, a serious disadvantage compared with the use of the dominant mode (mode of the lowest cut-off frequency), which is the TEM. In the first place, the circular modes require a larger size guide for transmission of any given frequency. Further, the attentuation tor any given frequency will in general be lower for the dominant mode. However, in the case of lthe dominant mode, as opposed to circular mode, rotation of the polarization pattern becomes a serious problem. Standing wave patterns in the guide become confused and uncertain where such rota-tion occurs because of differences of polarization at any given point between the incident and reflected wave. Further, even in Isystems designed for minimum reflection, the efficiency of the probe or other energy-extraction device at the load end, designed for the dominant mode, cannot be held constant if variation in .rotation occurs.

One type of structure which may be employed as circular waveguide capable of being flexed is suitably corrugated metal tubing. In such a type of tube, additional polarization preservation problems are encountered by reason of the presence of the corrugations, particularly if these are helical. Since the corrugations are prone to rotate the polar-ization, and Ksince the polarization rotation between the input and output ends will vary with bending of the guide, a simple flexible tube of circular symmetry is virtually useless from a practical standpoint as regards employment of the dominant waveguide mode. Further, it is impractical to incorporate 4in such constructions any elaborate structure for maintaining lthe direction of polarization.

The essence of the present invention lies in the discovery that circular waveguide, particularly flexibile waveguide, may be constructed to permit the use of the dominant mode without rotation of the plane of polarization of the propagated wave by constructions which are extremely simple and inexpensive to fabricate, but are nevertheless highly efficient in producing desirable waveguide characteristics for the dominant mode of circular waveguide propagation. The preservation of polarization of the dominant mode permits the propagation of transmissions of very high mode purity, since the employment of a frequency below the cut-off frequency of the TMm mode (the next higher mode) assures `that there can be no propagation yof modes other than the desired mode, and the irregularities which might otherwise produce add-itional modes of higher order cannot do so.

`One form of flexible metallic tubing is a tube formed from sheet material bent to circular form with .the ends slightly lapped to form a longitudinal seam which is welded or otherwise sealed along its entire length, the tubing being helically corrugated to provide flexibility. Corrugated tubings of this type have heretofore been employed for other purposes, such as the sheathing of coaxial cables. ln one type of such construction of tubing, the body material of the tubing is selected for its mechanical properties, such as steel, while an internal coating of a material of high conductivity, such as copper,

is employed to impart the desired inner surface conductivity. The essence of the present invention lies in the discovery that the addition to such tubing of suitable waveguide flanges, with the interior of the tubing unobstructed, provides a highly efficient, but relatively inexpensive, flexible waveguide for propagation of the dominant mode of circular waveguide transmission which compares very favorably in performance with the relatively expensive rectangular flexible waveguide. The exact theory of operation is not fully known, but the polarization of the dominant mode is preserved by the single circumferential deviation from circular electromagnetic symmetry which extends longitudinally along the interior of such a tube, this single circumferential deviation occurring at the lapped seam and being effective on the one hand because of the geometrical departure from circularity at the seam, and on the other hand because of the discontinuity in conductivity characteristics which occurs at the uncoated exposed edge of the original sheet material which bounds the seam.

For more complete understanding of the invention, reference is made to the embodiment thereof illustrated in the annexed drawing, and to the experimental data herein to be given concerning the performance and operation of the illustrated exible waveguide.

In the drawing:

FIGURE l is a view partially in longitudinal section and partially in elevation showing a waveguide constructed in accordance with the invention as attached to a radio-frequency source which is schematically illustrated in block form;

FIGURE 2 is a transverse sectional view taken along the line 2-2 of FIGURE 1; and

FIGURE 3 is an enlarged sectional view corresponding to a portion of FIGURE 2.

The flexible tubular portion of the illustrated waveguide 1l) comprises a tube 12 formed from metal sheet or strip into substantially circular cross-sectional form and having a lap seam at 14. The body 16 is of steel, the sheet or strip from which the body is formed'fseing coated with a copper coating 18 on one side (nowlK the interior of the tube). The opposite opposed edges 20 and 22, which bear no coating, are, after the illustrated formation of the tube, respectively the inner and outer edges of the seam 14 thus formed. The seam 14 is welded at 24 along the entire length thereof. In the welding process, the inner and outer edges which form the lap joint are slightly bent inward at 26 and 28 respectively to form a small ridge extending slightly into the circle otherwise formed by the tube, and thus constituting, both with respect to geometrical configuration (deviation from. circularity) and conductivity (the uncoated inner edge 20) a single circumferential deviation from circular electromagnetic symmetry extending longitudinally along the interior of the entire tube, the slight variation or alteration of the conductivity of the copper caused by oxidation in the welding operation also contributing to the effect. After formation of the tube in the manner described, the tube is helically corrugated by the formation, in a suitable rolling operation, of a groove 29 extending helically the entire length of the tube.

The ends of the tube 12 are brazed at 30 to ferrules 32 extending from flanges 34. In the illustration of the drawing, one of the flanges 34 is secured by means of bolts 36, and sealed by means of an O-ring 38 seated in a channel 40, to a flange 42 coupled, as schematically illustrated, to a microwave frequency source 44. It will of course be understood that this `illustration is highly schematic, either or both ends being adapted for any type of insertion in a microwave transmission system. The source 44 includes appropriate means for producing in the guide the TEM mode, with the diametric electric field vector which characterizes that mode appearing across the diameter which terminates on the seam.

The materials and dimensions of the tube are of course of great importance in determining its characteristics CII when the tube, internally unobstructed, is secured to suitable llanges to constitute a waveguide. Where the illustrated coated construction is used, the electrical characteristics of the body 16 are of secondary importance, and the material of the body 16 may be chosen primarily for its mechanical characteristics as regards flexibility when suitably corrugated and resistance to the type of deformation typified by dents and cracks, A typical material for construction of the body of the tube is stainless steel. It will of course be understood that partial utilization of the teachings of the invention may be employed by constructing the device shown in the drawing of a material which is itself of sufficiently high conductivity to permit low-attenuation waveguide transmission without an internal coating; such utilization of the teachings of the broader aspects of the invention, however, will in general produce waveguide structures less advantageous, since such structures will add materially to the cost and impair the mechanical stability, while at the same time eliminating the circumferential deviation from electromagnetic homogeneity which is produced by the uncoated internal edge of the lap seam where a coating is employed. Where a coating is employed, as `in the present illustration, the coating must of course be of high conductivity, but need be only of suficient thickness to constitute an adequate skin at the frequencies employed.

The pitch and depth of the helical corrugation is not greatly critical to proper operation as a waveguide, being selected primarily on the basis of mechanical flexibility characteristics which in themselves constitute no portion of the present invention.

Because of the effect on the lap seam in producing deviation from circularity around the circumference of the tube, the matter of wall thickness, or more properly of ratio of wall thickness to diameter, becomes of great moment. The use of excessively thin walls, in addition to producing inadequate mechanical strength, will also produce insufcient deviation from circularity of the interior to adequately hold the polarization of the dominant mode. On the other hand, the use of too thick a material, in addition to the mechanical problems thus introduced, will produce an internal shape which is no longer sufficiently circular to preserve a mode of propagation whose characteristics are well-known, but produces a type of propagation, complex and relatively unknown, which is far less satisfactory, particularly as regards coupling means for the microwave energy for the efficient insertion of the mode at one end of the guide and extraction at the other. When a lap seam is employed, the thickness of the lap should be from .C05 to .03 times the average diameter of the tube.

There may now be described the details of construction and the electrical performance of a particular construction of the device of the drawing which was built and extensively tested. In this construction, stainless steel of a thickness of 18 mils was copper-clad with a 2 mil coating. The bending at the weld was such as to produce in essence a flat strip of the thickness of the wall material and of a width of approximately 0.25 inch within the otherwise circular tube. The major inner diameter of the tube was 1.8 inches and the minor inner diameter was 1.57 inches, the pitch of the corrugations being 10.4 millimeters.

It was found that the standing wave ratio and attenuation were excessively high at frequencies below 4600 megacycles, the lower limit of high utility of the guide being approximately 1.10 times the theoretical low-frequency cut-off at a conventional circular guide of the same inner diameter, i.e., of an inner diameter corresponding to the minor inner diameter of the corrugated guide. In the region between 4600 and 5300 megacycles, the performance as regards standing wave ratio was quite comparable with that obtainable in other types of waveguide, averaging about 1.20 in the range from 4630 to 4660 megacycles, about 1.15 in the range from 498() to 502i) megacycles and as low as 1.05 in the range from 5250 to 5285 megacycles. In this same range, the attenuation varied from approximately 2.6 db per 100 feet at 4600 megacycles to as low as 1.9 db per 100 feet at approximately 5100 megacycles, but thereafter rising to about 2.7 db at 5 300 megacycles, apparently due to the loss of the power of maintaining polarization with the particular deviation from electromagnetic circularity employed. The useful range of frequencies therefore varies from approximately 1.10 times the cut-off frequency for the dominant mode to approximately 1.2.9 times the cut-'off frequency for the dominant mode. Since the upper frequency limit of desirable operation corresponds substantially with the cutoff frequency for the first higher mode, not only are the attenuation and standing wave characteristics favorable over the entire useful range, but in addition there is no possibility of appearance of higher modes, and the propagation through the guide accordingly consists solely of the dominant mode with uniform polarization throughout.

As previously indicated, the exact theory which produces the result is not known. It is found experimentally that the electric field vector terminates on the lapped zone or seam throughout the entire length of the guide. 'This observation is believed to be due to two cooperating effects. First, the geometrical factor produces a condition wherein the diameter of the tube which terminates on the seam is slightly smaller than the diameter normal thereto. The electric field vector accordingly tends to be aligned across the smaller diameter. The effect of the longitudinally extending discontinuity of the conductive coating is less clear, but it may be observed that the dine line of uncoated material appears parallel with the magnetic linx lines adjacent to the walls of the guide if the wave is travelling through the tube with constant direction of polarization, but has a component transverse to the magnetic flux lines of a Wave which might otherwise follow a spiral path.

It will be obvious lto persons skilled in the art that the teachings `of the invention can read-ily be adapted to waveguide structures differing in precise details from that herein illustrated and described. Accordingly, the scope of the invention should not be considered to be limited by the particular embodiment described, but shall be determined solely in accordance with the appended claims.

What is claimed is:

1. Flexible waveguide apparatus comprising a unitary strip bent to join its edge portions and form an internally unobstructed substantially circular tube and helically corrugated, means for introducing in said tube electromagnetic wave energy of a frequency below the cut-off frequency of any circularly symmetrical propagation mode therein, with a direction of polarization having an electric field vector normal to the wall at the joint between the edge portions, and means for maintaining the polarization direction constant as the energy is propagated through the tube, said direction-maintaining means includ-ing a continuous longitudinal weld of said joint having at least one internal surface of relatively high resistance.

2. The diexible waveguide apparatus of claim 1 wherein the joint is a lap seam, all inner surfaces of the tube other than the edge surface of the yinnermost edge portion being coated with a material of higher conductivity than the base material of the tube, and said edge surface being uncoated,

3. The iiexible waveguide apparatus of claim 2 wherein the base material is steel and the coating is copper.

4. The ilexible waveguide apparatus of claim l wherein at least the inner surface of the tube is copper, and an internal surface of higher resistance than the other portions of the interior is formed on the joint by oxidation of the copper.

5. The exible waveguide apparatus of claim l wherein the thickness of the strip from which the tube is formed is from .005 to .03 of the diameter of the tube, the joint being a lap seam, to form a circumferential deviation from circular symmetry.

6. The flexible waveguide apparatus of claim l wherein the edge portions of the st-rip are bent slightly inwardly at the weld.

7. The iiexible waveguide apparatus of claim l wherein the joint is a lap seam, the base material of the tube being steel of a thickness of from .005 to .03 of the diameter of the tube, all of the inner surface of the tube other than the edge surface of the innermost edge portion being coated with copper, and said edge surface being uncoated, the tube being deformed slightly inwardly at the weld, and the portion of the copper on the weld being oxidized to form a longitudinal surface of higher resistance than the other portions of the copper coating.

References Cited in the le of this patent UNITED STATES PATENTS 2,155,508 Schelkunoif Apr. 25, 1939 2,238,770 Blumlein Apr. 19, 1941 2,576,835 Hewitt Nov. 27, 1951 2,600,169 Lamb lune 10, 1952 2,783,440 Lovick Feb. 26, 1957 2,848,696 Miller Aug. 19, 1958 2,930,007 Anderson Mar. 22, 1960 

1. FLEXIBLE WAVEGUIDE APPARATUS COMPRISING A UNITARY STRIP BENT TO JOIN ITS EDGE PORTIONS AND FORM AN INTERNALLY UNOBSTRUCTED SUBSTANTIALLY CIRCULAR TUBE AND HELICALLY CORRUGATED, MEANS FOR INTRODUCING IN SAID TUBE ELECTROMAGNETIC WAVE ENERGY OF A FREQUENCY BELOW THE CUT-OFF FREQUENCY OF ANY CIRCULARLY SYMMETRICAL PROPAGATION MODE THEREIN, WITH A DIRECTION OF POLARIZATION HAVING AN ELECTRIC FIELD VECTOR NORMAL TO THE WALL AT THE JOINT BETWEEN THE EDGE PORTIONS, AND MEANS FOR MAINTAINING THE POLARIZATION DIRECTION CONSTANT AS THE ENERGY IS PROPAGATED THROUGH THE TUBE, SAID DIRECTION-MAINTAINING MEANS INCLUDING A CONTINUOUS LONGITUDINAL WELD OF SAID JOINT HAVING AT LEAST ONE INTERNAL SURFACE OF RELATIVELY HIGH RESISTANCE. 